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United States Patent i I 3,308,853 CONSTRAINED CERAMICS Donald L. OBrien, South St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Nov. 6, 1963, Ser. No. 321,860 4 Claims. (Cl. 138177) This invention relates to combinations of metal and ceramic having improved resistance to mechanical shock, to units for conducting hot gases and particularly to ducting units having ceramic passages and having improved resistance to mechanical shock.

In operations in which hot gases are conducted from place to place it is often desirable to take advantage of the thermal resistance of ceramic materials. In particular when heat capacity is also desirable, the bulk of ceramic is preferable to denser metals. A field in which these advantages can be particularly well utilized is in the construction of afterburners for automobiles.

The exhaust gases from internal combustion and diesel engines contain many noxious and obnoxious ingredients the precise composition varying with acceleration, deceleration and steady speed conditions. Among these materials are carbon monoxide, unburned or partially burned fuel and smoke or soot. The continual discharge of these materials into the atmosphere may, under certain atmospheric conditions, give rise to relatively high concentrations of these substances and may render the air Virtually unfit to breathe. Afterburners for automobiles, or more generally motor vehicles, are intended to effect substantially complete combustion of the components of exhaust gases to water vapor and carbon dioxide. This reaction can be induced in several ways and does not form a part of this invention. In all cases, however, the reaction is exothermic and the resultant gases are hotter than the normal exhaust gases from the engine. As a result of the high temperatures involved, roughly 800 to 1600 F. or even higher, ordinary metals are subject to very severe corrosion. Ceramic materials are the best choice for such conditions or for the protection of metals but they are normally subject not only to thermal shocks as the engine is started and stopped repeatedly, but also to severe mechanical shocks as when the car jounces on rough roads. Although these problems might be somewhat ameliorated by use of rather massive structures, this is not feasible in motor vehicles where there are practical limitations on size and weight.

One method for the use of bricks in massive structures is reviewed in Chemical and Engineering News for October 7, 1963, pages 5455. In this process the brick is cemented in position within a shell using spinal techniques. Such structures are obviously massive in size and although the brick is under compressive stress in several directions, only particular bricks having extremely high compressive strength and spinal mortars can be used.

It is one object of this invention to provide combination ceramic and metallic structures having improved resistance to mechanical shock.

It is another object of this invention to provide ducting having improved resistance to mechanical shock.

A further object of this invention is to provide ceramic structures for use in motor vehicles under conditions of high temperatures.

Yet other objects will become apparent from the disclosure herein.

In accordance with the above and other objects of the invention it has been found that ceramics having certain characteristics, when restrained under compressional stress over all available temperature conditions, possess unusual resistance to mechanical damage and shock. Available 3,303,853 Patented Mar. 14, 1967 temperature conditions will be understood to be the range of temperatures over which melting, fusion and creep do not affect the strengths of the materials employed. Ducting units constructed from ceramics with compressional stressing means are useful, for example, in engine exhaust afterburners where they may form parts of regenerative systems.

Having discovered empirically that certain ceramics are improved in their resistance to mechanical shock when thus compressed, I have investigated the possibilities of utilizing other ceramics and other metals. Ma-king reasonable assumptions on the basis of average properties as tabulated by handbooks, it appears that, in general, the most important characteristic of the ceramic when restrained by metals is a Youngs modulus of less than 300,000 p.s.i. This is true for ferrous metals having tensile strength of about 100,000 p.s.i. and COCfilClCIlt of thermal expansion of about 6X10 inches per inch per degree F. at a temperature of about 2000 F. when the average temperature of the ceramic is about 2500 F.

The coefiicient of thermal expansion of desirable ceramics generally tends to be about /3 that of e.g. steel, and the compressive strength considerably less than the tensile strength of the metal; of the order of less than 10 percent. Using the most unfavorable values of the properties of different metals, the desirable Youngs modulus for the ceramic is found to be about 50,000 p.s.i. but no one metal possesses all such extremes in properties. Accordingly it is found that for any combination of metal and ceramic constrained thereby, the Youngs modulus of the ceramic must be from about 50,000 to 300,000 p.s.i. when the coefiicient of thermal expansion is of the order of /3 that of the metal at very high temperatures, e.g. 1000 F. and higher. When the ceramic has a thermal coefiicient approximating that of the metal, values of Youngs modulus of about 1 to 1.8)(10 p.s.i. are suitable for the ceramic. Such values of thermal coefficients are, however, exceptional and may be discontinuous over a desired range of temperatures, for example, as the result of phase transformations which may occur in the ceramic. Concomitant with the high coefiicien'ts of thermal expansion of the ceramics is the necessity for vastly higher compressive strengths because otherwise the expansion of the ceramic, normally at a higher temperature than that reached by the metal, may result in actual crushing of the ceramic and subsequent loss of compression when cooled.

Stated differently, when the ceramic has a substantially lower thermal coefficient than the metals there is a relaxation of compression at elevated temperatures which must not, however, result in loss of compression. When thermal coefiicients are about equal, the ceramic being at a higher temperature, there is an increase in compression as temperatures are raised. The useful results of the invention are attained by a balance of properties as herein described.

Other considerations in the choices to be made are the melting points of metals, softening temperatures of ceramics and phase changes occurring in the ceramics over the intended temperature range. It is considered to be within the skill of the art to avoid difiiculties due to such considerations.

Broadly my invention comprises the combination of ceramic having Youngs modulus of from about 50,000 to 300,000 p.s.i. and by metallic means maintaining compressional stress thereon over the operative temperature range, the coefllcient of thermal expansion of the ceramic being about /3 that of the metallic means providing compressional stress.

The ceramics preferred in constructing the ducting units of the invention in combination with ferrous metals are 3 those having Youngs moduli of about .5 to 10x10 pounds per square inch. In particular I prefer zirconmullite ceramics prepared as described in the copending application of Kenneth E. Owens and Robert A. Hatch, Serial Number 258,298.

The invention is more fully illustrated by the embodiment shown in the accompanying drawings, wherein:

FIGURE 1 is a cross section of a casing having a hollow cylindrical ceramic shape therein and with retaining rings ready for insertion.

FIGURE 2 is a cross-section of an assembled ducting unit of the invention taken along the line AA in FIG- URE 3.

FIGURE 3 shows an end view of an assembled ducting unit of the invention.

FIGURE 4 is a perspective view of an assembled ducting unit of the invention having a multiplicity of parallel passageways.

FIGURE 5 is a perspective view with portions of one end cut away of a ducting unit of this invention adapted to use in the afterburner of my coworker Lemoine L. Johnson described in his copending application Serial Number 251,162.

FIGURE 6 is a cross section along line B-B of FIG- URE 5 of a ducting unit of this invention showing a multiplicity of ceramic members under compressional tension.

Referring to FIGURES 1-3, wherein the same parts have the same index numbers, a hollow cylindrical ceramic member 10, having Youngs modulus of less than about O.l 10 and substantially flat parallel end faces is placed in shell 12. End rings 14, shown disengaged in FIGURE 1 and in position in FIGURES 2 and 3, are inserted urged into contact with the flat ends of the cera-mic member 10 under suitable pressure and then welded in place as shown at 16 in FIGURES 2 and 3.

In FIGURE 4, a ceramic member 20 is provided having a multiplicity of parallel passageways and a heavy ceramic frame. The member is shown held under compressive tension in shell 22, which has openings 24 and stress bars 26, by means of retaining ring 28 continuously welded around the edges on the visible face. The rearward face may have a similar ring welded in place or the flat ends and back of the shell may be made by stamping and bending and the arcuate portions welded thereto.

The ceramic member is an elliptical prism cut parallel to the prismatic axis at equal distances along the longer elliptical axis. The ducting unit shown in FIGURE 4 is of a shape convenient for use in elliptical casings of the design commonly employed in automobile mufflers.

Referring now to FIGURES 5 and 6, views are shown of a ducting unit of the invention comprising a multiplicity of ceramic members of two types 30 and 32, those having like numbers being of the same type, held under compressional tension by foraminous retaining plates 34 and 35 welded or brazed to the constraining frame 36 as shown by lines 38.

The ceramic members of type 30 will be seen to be similar to those of FIGURE 4 having a corrugated ceramic core with many parallel passageways and an outer ceramic frame. The ceramic members of type 32 above one opening which is continuous therebetween and forms a chamber in which are located an ignition means 41, and an inlet 43, for the addition of supplementary air. These latter members may be formed from compressed board-like plates which are shaped and joined or from single pieces as hereinafter more fully described.

The frame 36 of the ducting unit comprises several integral pieces not forming a part of this invention which are shown so that the operation of the unit may be understood. In FIGURE 6, the top of the unit 40, is in position within flange 70. It may be held in position by any convenient means (not shown) such as screws, bolts, clamps or welding. This is broken away to show the posi tions of retaining plates and ceramic members in FIG- URE 5. The forward end of the unit is broken away in FIGURE 5 to show internal structures and the entire end is broken away in FIGURE 6. When the top of the unit 40' is inserted in the unit refractory packing material is packed tightly into the space above the ceramic members to minimize any possible blow-by of gases. The restraining frames further are shaped to fit tightly against the top. The other parts of the unit are most conveniently described in conjunction with the manner of operation.

This unit is intended for use in an afterburner for automobile exhaust gases. The forward end of the unit has a rectangular slot with basal lip 46. A valving device (now shown) is positioned on the lip in the slot so that raw exhaust gases may be directed either to port 48 in face 50 or to a similar port (not shown) in face 52 (most of which face is broken away). Assuming that the valve is set to direct the gases as shown by the arrow through port 48, they pass through an approximately semicircular passageway (not visible) and emerge from rectangular port 54 into the chamber before retaining plate 34. They pass from this chamber through retaining plate 34, which may include a perforated baffle plate not shown serving to effect more uniform distribution over the surface of forward ceramic member 30, through the small passageways into the central chamber. The small passageways act as a flame arrestor and also give up heat to the entering raw exhaust gases (having been heated in a previous regenerative cycle) so that the gases are combustible in the central chamber being ignited by ignition means 41. The burned exhaust gases emerge from the central chamber through the rearward ceramic member 30' to which they give up heat and pass through the rearward foraminous retaining plate 35 into the rear plenum space 56 and through a port (not shown) into the approximately semicircular duct 60 along the front side of FIGURE 5. This duct leads to the leg of the unit which is cut away and which has a port into the valve (not shown). In the assumed position of the valve the gases are directed into port 62 (shown in part) in partition 64 and into plenum 66 separated from the chamber before the forward restraining plate by partition 68. This plenum directs the gases into lower duct 75 which passes below the central structure of the unit and connects to outlet from which the gases are discharged to the atmosphere.

Means are provided changing the position of the valve ring means at regular intervals so that gas flow is reversed through the central chamber-and again restored after an equal interval. Heat is thus alternately given up to the rearward ceramic member and taken from the forward member and vice versa.

It will be apparent that there are many equivalent mechanical means for applying continuous compressive stress to the ceramic members of the ducting units of the'invention. Brazing or welding may be employed alternatively, depending on the metals involved. Stress bars may be employed, such as bolts which are locked to the proper tension. Steel rings may be urged against the flat ends by use of internally threaded caps of opposite screw by a double male member having hexagonal (or other flat faced) central section which can be drawn up to the proper tension using a torque wrench. For ease in replacement of the ceramic member, a threaded compressive means is desirable but because of the length of life achieved by ducting units of'the invention such construction is not necessary and units which are factory assembled and welded are preferable and more economical. This economy comes about because the welded construction is less expensive than more elaborate threaded constructions. Furthermore factory assembly insures greater adherence to the standards of construction necessary to achieve the advantages of the invention.

It is not necessary to employ massive metallic structures. It is only necessary that that portion which is under tension be somewhat more than sufficiently strong to withstand the tension. Thus sheet steel of the thicknesses normally fabricated in small units is fully satisfactory. The metal is normally at lower temperatures than the ceramic cores and is insulated from the very hot corrosive gases so that very little allowance for weakening from corrosion need be made.

Mechanically it is necessary that the surfaces of the ceramic member against which compressive stress is applied and the metallic faces providing the stress be substantially fiat and respectively parallel. This assures that the compressive stresses on the two ends will exactly oppose each other and that there will be no localized concentration of stresses, under eithercold or hot conditions, which might result in failure of the ceramic member. Such failure might occur, for example, if a slight elevation on one end received several times the stress intended. As a result of crushing of the peak, moreover, the compressive tension desired would be largely or completely lost.

It is advantageous that the ends be carefully finished, for example by careful lapping, and residual dust and grains be removed. Likewise to assure uniform loading or stressing, it is preferred that the ceramic be substantially homogeneous without large voids or bubbles and free from coarse aggregates, e.g. sand, gravel, and the like, which are sometimes necessary for the strength of certain ceramics.

From the disclosure herein made, the design of particular structures will be largely based on calculations by normal engineering methods. Thus it will be readily determined that the surface of the restraining ring in contact with the ceramic member should be sufiicient to support the total compressive force applied without exceeding the crushing strength of the member. The force applied preparatory to welding the ring in place will thus be calculated based on the area of engagement of the retaining ring for the ceramic member.

Although a cylindrical shell could be shrunk into the ceramic member, this is not necessary to achieve the benefits of the invention which are obtained when the ceramic member is under unidirectional compressive tension. Stated somewhat differently the force of compres sion must be applied in at least one direction to achieve the characteristic enhancement in resistance to mechanical and thermal shocks. This is of particular advantage when ceramic members are employed which may have different compressive strengths in different directions because of the internal structure. The differences may be of an order of magnitude. Thus in one block-like ceramic honeycomb structure, such as the core of the ceramic member of FIGURE 4, the compressive strengths were found to be 65 psi. normal to the planesof the flat sheets, 240 p.s.i. parallel to the fiat sheets and normal to the passageways and 750 psi. parallel to the passageways. Compressive tension is applied in the last direction although higher strengths than the above are obtained because of the heavier ceramic frame in this particular member.

As stated above the ceramics employed in the ducting units of the invention have Youngs modulus in compression of about 0.5 to 1.0 psi. The usual refractories and brick materials have significantly higher values, from about 0.4 to 1.2 10 are typical ranges of the modulus for silica bricks, building bricks and insulating bricks made of fireclay, silica, bauxite or kyanite.

A particularly useful and, as heretofore noted, preferred ceramic composition is found to be a combination of zircon and mullite. This is very conveniently obtained by firing compositions produced by the teachings of my coworkers, Kenneth E. Owens and Robert A. Hatch as set forth in their copending application Serial Number 258,298. According to their teachings compositions are prepared which can be worked to desired shapes in the green, unfired state and are then fired to give ceramic composition of desired mineral species. Certain of the properties of the ceramics are determined by the mineral species, others are variable Within some limits depending upon the method of working employed. The following example is provided as illustrative of the practice of the present invention in making the ducting unit shown in FIGURES 5 and 6 as an embodiment of the invention.

Two separate batches of organic fiber are beaten independently, each consists of 15 lbs. scrap of highly beaten paper (e.g. map overlay tracing paper) and 45 lbs. of bleached western long-fibered wood pulp in 200 gals of water. Beating is continued with the beater roll down for 20 minutes to disperse the scrap highly beaten paper and then for a further 15 minutes after adding the wood pulp. Each batch is transferred to the stock chest of a Fourdrinier machine using a further 50 gallons of water per batch. The mineral materials and inorganic fibrous materials are mixed into water in a Holland-type beater and transferred to the stock chest in three portions as follows.

Portion l 2 3 Water (gals) 150 150 150 Kaolin (lbs) 37. 5 37. 5 75 Alumina (lbs) (1-15 micron) 37. 5 37. 5 75 Zircon (lbs) (1-15 micron) 187. 5 187. 5 375 Alumino-silicate fibers (lbs.) (2-5 micron diam. ll. 3 11. 3 22. 6 Glass fibers (lbs.) (Tradename Fiberglas N o. 106) 3. 8 3. 8 7. 6

To the batch in the stock chest are then added 75 lbs. (dry weight) of 40 percent solids butadiene-acrylonitrile latex (available under the tradename Hycar No. 1562) diluted with 150 gallons of water and .cc. of a water dispersible silicone antifoaming agent (such as the commercially available Anti-Foam 60) to decrease foaming. The slurry thus obtained contains about 10%11% solids. It is pumped from the chest and diluted with water to about 3.5% solids (by adding twice its volume of water) and then flocculated in the mixing box, using a solution of 7 quarts of 2% polyethylenimine and about 2 pints of antifoaming agent diluted to 5 gallons with water, metered into the slurry at about 315 cc. per minute.

The flocked suspension is termed a pulp. It may be dewatered by several different methods. For making sheets for the honeycomb structure, the pulp is formed into a paperlike greensheet using a Fourdrinier machine with 70 x 56 wire screen at an initial speed of 16 feet per minute, increased to about 22 feet per minute after 70 minutes. The total time for the forming operation (to exhaustion of the stock) is about 170 minutes. Vacuum boxes and vacuum couch roll are maintained at 13 and 24 inches Hg vacuum respectively. It passes successively through a first pressing section (5 lbs. per lineal inch between nips), a second press section (100 lbs. per lineal inch) and then through 8 heated drying rolls, each 2 feet in diameter, the temperature of drying increasing incrementally from about to about 210 F. As formed the greensheet is about 21 mil caliper and it is densified using 375 lbs. per lineal inch pressure to about 1516 mil caliper. During operation of the Fourdrinier, losses of mineral materials are determined by analysis of samples of the white water passing through the screen. Random variation is found which is apparently associated only with the extent to which mixing is not entirely eflicient. Actual white water losses are very low (Well below 10%). The total web is 937 yards of 24- inch width in two large rolls of approximately 380 lbs. basis weight.

The honeycomb structures forming the cores of the terminal ceramic members are made up using corrugated and flat sheets. Two are made as described below.

A quantity of the above rolls is cut into manageable sizes and passed between mating corrugated rolls to give 7 corrugations about inch high .per inch. Sheets of the corrugated and flat material are cut to about 2 /2 x inches and formed into a core blank by laying up alternate sheets of each type to a total thickness of about 3% inches. Adhesion of the ridges of the corrugated sheets to the fiat surfaces is achieved by painting a thin suspension of the same composition in water on the one surface before applying the next superior sheet. The cores are fired at about 1660" C., are cut square at the ends (parallel to the passageways; normal to the flat sheets) and cut arcuate on both sides (parallel to passageways) to be 3 inches wide at the center and 2 inches wide at the ends.

The solid ceramic frame is next applied to the core. A thick paste of the same green, i.e. unfir-ed, mixture is made up using scrap pieces of unfired sheets and pieces of the sheet from the large rolls. It is, of course, also possible to employ the sheet material as it emerges from the Fourdrinier, before calendering and drying, or the pulp with only sufiicient dewatering to give a workable consistency. The thick paste of the green material is applied to the cut ends and arouate sides of the core. This can be done either by trowelling or the core may be placed in a form and the paste packed around it to give a block about 4 /2 x 7 inches and about 2% inches thick. Insofar as possible the thick paste is not applied to the faces of the core because it may plug some of the passageways. This block is fired at about 1660 C. It is cooled and cut to final size to fit the frame. The widest point between arcuate sides is about 4 inches narrowing to about 3 inches at each end. The length between ends is 6 /2 inches. The faces are then cut or ground to be parallel and lapped free from protuberances so that the thickness is about 2%. inches full.

The central ceramic members are made of raw inch thick plates molded and pressed to the desired shapes and approximate dimensions using a thick paste as employed above or from a multiplicity of fiat sheets laminated together. After firing the plates are cut and fitted to the desired form.

The metal frome work is prepared from sheet steel and one retaining plate is brazed in position. The ceramic members are inserted in proper sequence taking care to avoid inclusions of substantial dimensions between the different members, and the other retaining plate is inserted. A force of about 150' lbs. per square inch of contact between the edge of the retaining plate and the adjacent ceramic member is applied by suitable clamps or screw jacks and the retaining plate is brazed in that position. The applied force is then released. Elastic compression of the ceramic exerts a tensioning force on the metal frame so that the two forces balance one another. It will be realized that the actual amount of compression is quite small. The top plate is secured in position using only suflici-ent packing to prevent by-passing of gases and the unit is assembled together with the other mechanical accessories in a test car. The car is test driven for a distance of 12,000 miles. Inspection shows that there has been no significant deterioration of the ceramic members due to either the thermal or mechanical stresses occurring as a result of operation. In further test driving some evidence of mechanical deterioration is first evident after about 20,000 miles.

It will be recognized that the regenerative nature of the unit is such that the terminal ceramic members go through frequent thermal cycles from about 700 to about 1500 F. This cycle requires about 8 seconds in the test model described. These thermal stresses are further associated with severe mechanical vibration.

In contrast to the above ducting unit of the invention, a mechanically similar unit, in which the ceramic members are positioned only by retaining clips which do not give any compressional tension, is found to begin to deteriorate as a result of similar stresses after about 5000 to 6000 miles. Likewise when the ceramic members are constructed from materials having very low or very high coefficients of expansion, or insufficient compressibility, i.e. higher Youngs moduli, it is found that deterioration occurs after only a few thousand miles of driving.

What is claimed is:

1. In combination, a refractory ceramic structure having Youngs modulus of from about 50,000 to about 300,000 psi. and metallic means at least unidirectionally compressing said structure with force less than the compressive strength thereof throughout a temperature range from ambient temperatures to elevated temperatures below the lesser of the melting point of said metallic means and the sintering temperature of said refractory ceramic structure, said refractory ceramic structure being further characterized by coefficient of thermal expansion of about one-third the corresponding coefficient of the metal of said metallic means.

2. The combination of claim 1 wherein the ceramic structure is composed of zircon and mullite in cornbination and the metal is a ferrous metal.

3. A ceramic ducting unit consisting essentially, in combination, of a ceramic core having Youngs compressional modulus not more than about 1x10 pounds per square inch and a metallic restraining frame axially retaining said core in compression on substantially fiat parallel surfaces at less than the crushing strength of said core, said core having a coefiicient of thermal expansion substantially one-third of the coeflicient of thermal expansion of the said restraining frame and having at least one axial passage therethrough.

4. In combination with a ceramic ducting piece adapted for use under conditions of thermal and mechanical stress and characterized by a Youngs modulus of elasticity of not more than about 1 10 pounds per square inch, metallic means at least unidirectionally compressing said ceramic ducting piece with a force of from about 20 to percent of the breaking strength thereof under thermal conditions over the range from ambient temperatures to the lesser of the softening temperature of said ducting piece and of the metal of said metallic means.

References Cited by the Examiner UNITED STATES PATENTS 2,745,437 5/1956 Cornstock 138177 X 2,759,723 8/1956 Crespi -a 263--46 X 3,025,175 3/1962 Aldred 106-57 LAVERNE D. GEIGER, Primary Examiner.

T. L. MOORHEAD, Assistant Examiner. 

3. A CERAMIC DUCTING UNIT CONSISTING ESSENTIALLY, IN COMBINATION, OF A CERAMIC CORE HAVING YOUNG''S COMPRESSIONAL MODULUS NOT MORE THAN ABOUT 1X10**5 POUNDS PER SQUARE INCH AND A METALLIC RESTRAINING FRAME AXIALLY RETAINING SAID CORE IN COMPRESSION ON SUBSTANTIALLY FLAT PARALLEL SURFACES AT LESS THAN THE CRUSHING STRENGTH OF SAID CORE, SAID CORE HAVING A COEFFICIENT OF THERMAL EX- 